Expanded Granular Sludge Bed Reactor EGSB

Anaerobic sludge bed (and in particular the EGSB) reactor systems can be started up within a few days with granular seed sludges, and they may be applied across a wide range of conditions and strengths of wastewater (Lettinga, 1996). EGSB systems are particularly suited to low temperatures (10°C) and very low strengths (very much smaller than 1000 mg/L) and for the treatment of recalcitrant or toxic substrates.

New insights into the anaerobic degradation of very different categories of compounds, and into process and reactor technology will lead to very promising new generations of anaerobic treatment systems (Lettinga et al., 1997). These concepts will provide a higher efficiency at higher loading rates, are applicable for extreme environmental conditions (e.g. low and high temperatures) and to inhibitory compounds. Moreover, by integrating the anaerobic process with other biological methods (sulfate reduction, micro-aerophilic organisms) and with physical-chemical methods, a complete treatment of the wastewater can be accomplished at very low costs, while at the same time valuable resources can be recovered for reuse.

Anaerobic treatment of chemical and brewery wastewater with a new type of anaerobic reactor, the Biobed® EGSB reactor was reportedly very effective (Zoutberg and Frankin, 1996). The new ultra high loaded type of anaerobic reactor is in full-scale implementation to treat wastewaters from the chemical industry and the brewery. The chemical factory involved is Caldic Europoort in the Netherlands. In this factory formaldehyde is produced from methanol. The wastewater is characterized by high concentrations of these compounds (formaldehyde up to 5 g/L and methanol up to 10g/L). Due to the special configuration of the employed EGSB reactor, it is possible to acquire removal efficiency for both compounds of more than 99%. At the brewery plant, the Biobed® reactor was installed before an existing aerobic treatment. In this application, the reactor served as a "COD remover", which results in a substantial decreased COD load to the aerobic post-treatment causing lesser sludge production and lesser energy consumption. It is possible to treat wastewater containing toxic but degradable chemical compounds.

The Biobed® EGSB system is able to overcome shortcomings of the upflow anaerobic sludge blanket reactor in the chemical industry (Zoutberg and de Been, 1997). The most striking feature is the growth of biomass in a granular form, similar to the UASB granules: no carrier material is used. The process is especially suitable to treat wastewater that contains compounds that are toxic in high concentrations and that only can be degraded in low concentrations (chemical industry). It is also possible to operate the reactor as an ultra-high loaded anaerobic reactor

(to 30kgCÜD/m3-d) for applications in other sectors of the industry (e.g. brewery, yeast, sugar, corn ethanol production etc.).

Psychrophilic (8°C) anaerobic treatment of partly acidified waste water is also applicable using an EGSB system (van Lier et al., 1997) as the average CODsoluble and VFA-COD removal efficiencies were 97 and 90%, respectively. Besides, psychrophilic (2 to 20°C) anaerobic treatment of low strength synthetic and malting wastewater was also possible (Rebac et al., 1999). The COD removal efficiencies found in the experiments exceeded 90% in the single module reactor. When a two module EGSB system was used at the temperature range 10-15°C, soluble COD removal and volatile fatty acids removal of 67-78% and 90-96% was achieved, respectively. The mineralization of anthranilic acid as the only carbon and energy source was possible at low influent concentrations (Razo-Flores et al., 1999).

Mesophilic conditions at 35°C to treat slaughterhouse wastewater in an EGSB system appear to be a feasible option (Nunez and Martinez, 1999). The average COD removal efficiency was 67%. Total suspended solid (TSS) removal was 90%. Fats were 85% removed and no accumulation of fats on the sludge was observed. The specific methanogenic activity of the sludge was about 3 times higher than that of the sludge inoculated into the reactor. The sludge activity did not change significantly after one year of operation.

Thermophilic sulfite and sulfate reduction offers good prospects as part of an alternative technology to conventional off-gas desulfurization technologies (Weijma et al., 2000). Methanol can be efficiently used as electron and carbon source to obtain high sulfite and sulfate elimination rates in thermophilic bioreactors.

In Germany, there are currently 125 full-scale anaerobic treatment plants treating industrial wastewater from beet sugar, starch, pectin brewery, distillery, vegetable, and potato processing. The first EGSB reactor at a German potato-processing factory as well as the first municipal wastewater treatment plant combined with a separate anaerobic stage to treat a wastewater mixture from several small factories, demonstrated a successful experience (Austermann-Haun et al., 1999).

A comparison of the behavior of EGSB and UASB reactors in diluted (e.g. ethanol, diluted beer) and concentrated (e.g. coffee) wastewater treatment has been made. There were no big differences in the removal rates during the operation with coffee wastewater. It was likely that in this effluent the process is limited by the reaction kinetics instead of by the mass transfer, due to the complex nature of the waste. With diluted beer, EGSB reactor indicated a better performance than the UASB (Jeison and Chamy, 1999).

The Development of UASB and EGSB

The granular sludge-based UASB and expanded granular sludge bed (EGSB) processes gradually take a large portion of applications. Although UASB remains as the predominant technology in use, EGSB type processes are increasingly gaining more popularity.

Figure 3.1 shows the development regarding changes of process selection in granular sludge systems over time. It is apparent that the traditional UASB system is gradually being replaced by EGSB type systems. This is due to the effectiveness and competitive advantages of the EGSB system.

The data from Fig. 3.2 evidence that the design load for EGSB systems is approximately double that of the UASB process, which results in a

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Fig. 3.1. Total number of UASB plants (open bar) and EGSB plants (filled bar) (Frankin, 2001).

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Fig. 3.2. Design loading rates for various processes (Frankin, 2001).

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EGSB Hybrid Low rate UASB

competitive advantage over lower loaded systems. It should however be noted that the data presented represents approximately 50-60% of total anaerobic systems installed and contribution of EGSB and internal circulation (IC) systems may be relatively high in the current database relative to total number of systems installed.

From Fig. 3.2, it follows that the average (design) loading rate for the 198 EGSB plants in the database is somewhat over 20 kg COD/m3-d. This is two times higher than the average loading rate for 682 UASB designed at 10kg COD/m3-d.

The higher design loading rates determine lower cost for reactors, which contributes to the overall cost competitiveness of the process. This explains further the trend as observed in Fig. 3.1.

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